Merge #16001: Give WalletModel::UnlockContext move semantics

0b09a57 Give WalletModel::UnlockContext move semantics (Pieter Wuille)

Pull request description:

  WalletModel::UnlockContext seems to implement "move upon copy" semantics; with C++11 this can be done more safely using move semantics (making attempts to actually copy fail instead).

  Not a big deal if this isn't worth review time.

ACKs for commit 0b09a5:
  Empact:
    utACK bitcoin/bitcoin@0b09a57
  jonasschnelli:
    utACK 0b09a57
  jb55:
    utACK 0b09a57

Tree-SHA512: f827856586afd03666c2d9f50320776afb3dd511ac1bcd293b330f015acd1588551b163dccc97b1351301e3295f4c74d90e5754bcee89faeadf6437d7db165c8

Author: meshcollider

src/qt/walletmodel.cpp

@@ -482,7 +482,7 @@ WalletModel::UnlockContext::~UnlockContext()
     }
 }
 
-void WalletModel::UnlockContext::CopyFrom(const UnlockContext& rhs)
+void WalletModel::UnlockContext::CopyFrom(UnlockContext&& rhs)
 {
     // Transfer context; old object no longer relocks wallet
     *this = rhs;

src/qt/walletmodel.h

@@ -194,15 +194,18 @@ class WalletModel : public QObject
 
         bool isValid() const { return valid; }
 
-        // Copy operator and constructor transfer the context
-        UnlockContext(const UnlockContext& obj) { CopyFrom(obj); }
-        UnlockContext& operator=(const UnlockContext& rhs) { CopyFrom(rhs); return *this; }
+        // Copy constructor is disabled.
+        UnlockContext(const UnlockContext&) = delete;
+        // Move operator and constructor transfer the context
+        UnlockContext(UnlockContext&& obj) { CopyFrom(std::move(obj)); }
+        UnlockContext& operator=(UnlockContext&& rhs) { CopyFrom(std::move(rhs)); return *this; }
     private:
         WalletModel *wallet;
         bool valid;
         mutable bool relock; // mutable, as it can be set to false by copying
 
-        void CopyFrom(const UnlockContext& rhs);
+        UnlockContext& operator=(const UnlockContext&) = default;
+        void CopyFrom(UnlockContext&& rhs);
     };
 
     UnlockContext requestUnlock();